While the exact mechanisms of ischemia/reperfusion injury remain unclear, our laboratory and others have shown that inhibition of complement activation (e.g., sCR1 or C1 esterase inhibitor), depletion of complement components (e.g., cobra venom factor) and complement component (e.g., C3 and C4) deficient mice reveal an important role of complement in ischemia/reperfusion injury. In the last funding cycle, we showed that inhibition of mannose binding lectin (MBL) protected the ischemic gastrointestinal system from the primary ischemic event. In contrast, the pulmonary system, which takes a second hit from the primary ischemic gastrointestinal tract, was not protected. Complement activation in the lung was observed in C1qa- or MBL-deficient mice, but not in C2/factor B deficient (KO) mice. Addition of C2 restored the complement activation and tissue inflammation/injury in the C2/fB KO mice following gastrointestinal ischemia/reperfusion (GI/R). These data suggest that a C2 dependent, but C1q or MBL independent, process is responsible for the complement activation, inflammation and injury following GI/R. Preliminary data suggest that another lectin, ficolin-A is the initiating molecule responsible for complement activation in the lungs following GI/R. Recent publications also showed a C2/C4 bypass mechanism involving MBL may be responsible for complement activation via the alternative pathway. Preliminary studies demonstrate biological evidence of this bypass mechanism in vitro and several potential molecular mechanisms involving the MBL complex and the coagulation system. Studies by others show several different interactions with the coagulation system and complement. Using the FeCl3 model of arterial thrombogenesis, we observed that thrombogenesis was absent in MBL null mice, yet present in C2/fB null mice. We show that trauma patients demonstrate a link between complement activation and coagulopathy with a significant decrease in MBL levels observed early on. At least one of these interacting links involves the direct activation of the alternative pathway by the MBL complex, bypassing the need for C2 or C4. Our preliminary data demonstrate multiple interactions of lectin complexes with coagulation proteins and may explain why inhibition of complement has proven unsuccessful in clinical studies for CABG and CAD, warranting a better understanding of the complex interactions of complement with the coagulation system. In this competitive renewal, we will continue to investigate the molecular mechanisms involved in complement activation during oxidative stress of endothelial cells in vitro and in vivo and its interactions with the coagulation cascade. The general aim of this application is to characterize the molecular mechanisms governing lectin induced activation of coagulation following endothelial oxidative stress. Studies in this renewal will investigate the molecular mechanism of complement activation and interactions of complement with coagulation proteins. We will also be characterizing the potential use of a novel complement inhibitor that was recently discovered, cloned and expressed since our last submission, and is part of the lectin complex.